This invention relates to a method of manufacturing a transparent substrate for a mask blank suitable for a short wavelength region where the exposure wavelength is 200 nm or less, and further relates to a mask blank manufacturing method and an exposure mask manufacturing method.
In recent years, following the miniaturization of semiconductor devices, exposure light sources used in the photolithography technique have been changing to an ArF excimer laser (exposure wavelength: 193 nm) and an F2 excimer laser (exposure wavelength: 157 nm) so that reduction in exposure wavelength has been proceeding.
When the exposure wavelength becomes 200 nm or less, the depth of focus of an exposure apparatus becomes extremely small. Accordingly, if an exposure mask is deformed to reduce its flatness when the exposure mask is set in the exposure apparatus by vacuum suction or the like, the focus position is shifted upon transferring a mask pattern of the exposure mask onto a semiconductor substrate as a transfer target. In this manner, transfer accuracy is often reduced.
In order to solve this problem, JP-A-2004-46259 (hereinafter will be referred to as a patent document 1) discloses a technique in which the flatness of a mask blank for producing an exposure mask, when it is set in an exposure apparatus, is estimated by calculation through simulation and an exposure mask is produced from the mask blank in which estimated flatness is excellent.
Specifically, a mask blank is produced by forming a light-shielding film (an opaque film) on a transparent substrate, and the surface shape (one of four kinds, i.e. convex shape, concave shape, saddle shape, and semicylindrical shape) of the main surface of the mask blank and the flatness of the mask blank (a difference between the highest point and the lowest point of the main surface of the mask blank with respect to a certain reference plane) are derived by measurement. Then, from the thus derived flatness of the mask blank and a structure of a mask stage of an exposure apparatus, the flatness of the mask blank, when it is set on the mask stage of the exposure apparatus, is derived through simulation by the use of a finite element method or the like. When the flatness of the mask blank derived through the simulation satisfies a specification, an exposure mask is produced from such a mask blank.
However, in the technique of patent document 1, those data that are used upon carrying out the simulation for deriving the flatness of the mask blank when it is set in the exposure apparatus are the flatness (the difference between the highest point and the lowest point of the main surface of the mask blank with respect to the certain reference plane) and the surface shape (one of four kinds, i.e. convex shape, concave shape, saddle shape, and semicylindrical shape).
When the main surface of the transparent substrate is precision-polished, its surface shape is complicated, i.e. for example, having waviness or in combination of convex and concave shapes, and therefore, there is a case where the surface shape does not correspond to any one of the foregoing four kinds. Accordingly, even by forcibly applying the complicated surface state of the main surface to the flatness of the mask blank and the simple surface shape (convex shape, concave shape, or the like) to derive, through the simulation, the flatness of the mask blank when it is set in the exposure apparatus, the derived flatness may not agree to a flatness of an exposure mask produced from the mask blank when the exposure mask is actually set in the exposure apparatus.
Further, in the technique of patent document 1, the data (surface shape and flatness) that are used upon carrying out the simulation for deriving the flatness of the mask blank when it is set in the exposure apparatus are those of the mask blank having the light-shielding film formed on the transparent substrate.
After a mask blank is produced by forming a light-shielding film on a transparent substrate, if the flatness and the surface shape of this mask blank are measured, quite a large number of particles adhere to the light-shielding film to cause defects. When the exposure wavelength reaches a short wavelength region of 200 nm or less, the safety margin is reduced in a specification determining the size and number of defects and a specification of optical properties (e.g. deviation of transmittance etc. from design values and in-plane variation in transmittance etc. on the main surface). As a consequence, the mask blank may not satisfy those specifications.
Further, in the case where a film stress of the light-shielding film of the mask blank is large, when an exposure mask is produced by patterning the light-shielding film, a difference possibly occurs between the flatness of the mask blank derived through simulation and the flatness of the exposure mask when it is actually set in an exposure apparatus, depending on the shape of a pattern of the light-shielding film, the occupation ratio of the pattern of the light-shielding film on the main surface of the transparent substrate, and so on, particularly when the light-shielding film is reduced. Accordingly, there is a possibility that the flatness cannot be accurately estimated.